SQ was a program used in the early 1980s on both DOS and CP/M computer systems to compress files so they use less space. Files compressed by SQ are identified by changing the middle initial of the extension to "Q", so that text files ended with the extension .TQT, executable files ended with the extension .CQM or .EQE, documents with .DQC, etc.
Groups of files were often combined into an archive using the LU program, which created .LBR files containing all the files needed for a particular group, such as all the files needed to install an application. Typically such files were either individually compressed or the LBR archive was itself compressed with SQ.
With the development of the ARC program and its ARC archive file format, SQ essentially became obsolete on most systems, except CP/M, which lacked an ARC port for several years. On CP/M systems, the CRUNCH compression program was written to implement the LZW algorithm and plug the gap.